1. Field of the Invention
The present invention relates to an optical device, particularly to an optical device in which there is spin coated nanoparticle on its light emitting surface to increase its Light Extraction Efficiency and the manufacturing method thereof.
2. Description of the Prior Art
Current semiconductor luminous element, such as light emitting diode, has the advantages such as long life, small volume, high vibration resistance, low heat generation and low power consumption etc., which is already widely applied in cell phone display backlight module, vehicle lighting system, decoration lamp and remote control field as the indicator or light source. Moreover, because the luminous efficiency of light emitting diode has been raised constantly, the white light emitting diode has gradually substituted the conventional fluorescent lamp and incandescent bulb on applications such as the light source of scanner, backlight source of liquid crystal display or lighting equipment etc.
Please refer to FIG. 1A. FIG. 1A illustrates the cross-sectional diagram of prior-art top emission light emitting diode. As shown in the FIG. 1A, the luminous element 10 (i.e. light emitting diode 10) includes a substrate 11, a N-type semiconductor layer 12, a multiple quantum well layer 13, a P-type semiconductor layer 14 and two electrodes 15. Generally speaking, in order to conduct the P-type semiconductor layer 14 and the N-type semiconductor layer 12 to operate the light emitting device 10, one electrode 15 is formed on the P-type semiconductor layer 14, and another electrode 15 is formed on the N-type semiconductor layer 12. In addition, before forming another electrode 15, it is necessary to partially etch the P-type semiconductor layer 14, the multiple quantum well layer 13 and the N-type semiconductor layer 12 on the light emitting device 10. Then, another electrode 15 is formed on the exposed part of N-type semiconductor layer 12. After recombining the electron and hole injected by semiconductor layer, the light beam L is generated on the luminous area, and emitted from the transparent electrode on the P-type semiconductor layer or substrate (please refer to the flip chip LED shown in FIG. 1B). The common materials used for making LED include various III-V Group compounds, such as the aluminum gallium indium phosphide (AlGaInP), and the gallium nitride (GaN), wherein the gallium nitride LED is grown on the sapphire substrate.
Theoretically, the efficiency of LED is influenced by two factors, such as the Internal Quantum Efficiency (IQE) and the Light Extraction Efficiency (LEE). The IQE is determined by the property and quality of material. The LEE is the ratio of light emitted from device to the ambient air or encapsulated epoxy resin, which is an important factor for influencing the efficiency of LED. The behavior if light in LED is determined by the LEE mainly. Because the LED material is the high refractive index medium, when the light wants to penetrate LED, it will encounter the problem of total internal reflection, so that only about 8% of luminous energy can leave the surface of LED actually, which is the LEE. Thus, if the Light Extraction Efficiency (LEE) of LED can be raised, the whole luminous efficiency of LED can also be raised significantly.
At present, many methods have already been proposed for increasing the Light Extraction Efficiency (LEE) of LED. The most common method is to make a coarse structure or make other periodic structure to reduce the total internal reflection effect on light emitting surface of LED, in order to increase the Light Extraction Efficiency (LEE). However, these methods almost need complicated process of etching or lithography process, the former process will destroy the electrical property of LED to influence the efficiency of device, and the later process will increase the manufacturing cost greatly.